Multiple-roast chloridizing process



Patented Oct. 13, 1925.

UNITED STATES ATENT OFFICE.

OSCAR B. HOFSTRAND, F SALT LAKE CITY, UTAH.

MULTIPLE-ROAST CHLO'RIDIZING PROCESS.

11' 0 Drawing.

This discovery-relates to a multiple-roast chloridizing process for the extraction of values from metalliferous ores and the like. 15 The process is characterized by a series of successive roasting stages, and its principal objects are First. To bring about an increased extraction of values from certain ores which contain. substances that ordinarily interfere with the efficient application to'those ores, of the chlorination process as commonly practised.

Second. To achieve a high percentage of extraction with the machinery and apparatus now in use.

Third. To provide a process which shall be simple, economical and eflicient.

Fourth. To provide a process which may be practised by the ordinary mill organization without the necessity of employing specially trained, expert talent.

In attaining the objects outlined above, the ore to be treated is given two or more roasts, each roast being applied to the ore in approximately the same manner as'in the ordinary chlorination process where the ore is given only one chloridizing roast, and the practice of which is well known. By the roasts additional to the first one, there is accomplished an increased extraction of values, from those ores for which this process is designed, to a degree that is never obtainable by a single roast.

A typical method of practicing the well known process which has been In use for many years, will be described first. Briefly, the ore to be treated, is crushed to pass a to give 2 to 3% sulp Application filed April 18, 1922. Serial No. 555,360.

screen of about four mesh, after which the ore is stored in a suitable bin. A second ore which is selected with re ard to a pre-determined content ofso-called fire-sulphurl, the nature of which will presently be described, is similarly crushed, and stored in a separate bin. In a third bin is stored common salt which usually comes to the mill in large lumps but is crushed to a suitable size before being placed in the bin. Slack coal may be stored in a fourth bin, but of course requires no special grinding. These bins are located in what is known as the mixing room, and each bin may be provided with a .gate and an adjustable feeding device which latter may be regulated to discharge onto a conveyor, the proper proportion of each of the substances named. The conveyor may "be any one of several different kinds, though what is known asa belt-conveyor is usuall preferred. I

' Di erent ores require to be mixed with different amounts of the other substances,

the usual proportions bein about as follows: Salt'6 to 8% sulphi e ore suificient hur, coal 1 to 2%. Theconveyor carries these ingredients to an ele vator, and the latter lifts the mixture to a screen of about 6 to 8 mesh. The undersize goes through the screen while the oversize" is returned to the crushing room to be recrushed and again brought to the screen. During the conveyin screenin and recrushing operations, t e ingredients are thoroughly mixed witheach other and the comparatively uniform mixture discharged from the screen as undersize, drops into a binwhich serves the roasting furnace. As the ore drops into the bin, water is added to the extent of 5 to 7%. The moistened mixture is now ready for the furnace which ordinarily may be of various 90 designs, but the one which has so far been found most satisfactory for chloridizingpurposes generally and also in the present process, is known as the Holt-Dem chloridizing furnace. As soon as one charge of ore is completely roasted, the

lower part of the charge is dropped through the grate,-the' upper and hottest part of the charge being left upon the grates. A fresh charge of ore. is then dumped on the fire, air under pressure is supplied below the ates and is forced upwardthro-ugh the c arge of ore until all the ore on the grates is roasted to the required degree.

After the chloridizing roast is completed, the'ore is dumped into leaching vats, where a solution of common salt is allowed to percolate through the roasted ore until all the chloridized values have gone into solution. The pregnant solution is then removed from the tanks and is precipitated to remove the values. Silver may be precipitated on copper, and copper on iron; while the lead may be precipitated by a current of electricity, or on sponge iron. The tailings from the brine leaching operation may be cyanided to'recover such a part of the gold as has not been recovered by the former treatment.

, When an oxid or sulphid ore is subjected to a chloridizing roast as outlined above, followed by a subsequent treatment such as cyanide, hypo-sulphite, neutral or acid brine-leaching, it has been found that a content of calcium carbonate or magnesium carbonate in the ore, interferes greatly with the recovery ofthe metals, especially with silver. It is for this class of ores that my improved process is rendered especially valuable. In carrying out this improved process, the ore, after coming from the chloridizing furnace, following the first roast, is conveyed back to the mixing room where it is mixed with a fresh quantity of salt and sulphur, or sulphur-bearing ore, and the roasting operation as above described, is repeated. Sometimes three or more roasts may be required, the ore for each additional roast being mixed with a fresh quantity of salt and sulphur, or sulphurbearing ore, as described. After the final roasting, the resulting mass is brought into I solution and precipitated in. the usulal manner.

As an example illustrating the advantages of the present process over the ordinary chlorination process, its application to ores high in calcium and magnesium carbonates, ma be mentioned. First, it should be un erstood that on an ore free from calcium and magnesium carbonates, a single chloridizing roast shows a recovery of old and silver of and90% respective y. In a single chloridizing roast on an ore high in calcium or magnesium carbonates, therecovery of gold and silver is only 65% and 70% respectively, and therefore, such treatment is unprofitable. With the sameore, however, using the double or multiple chloridizing roast, as practiced in the present process, the recovery-is found to be approximately 83% of the gold and 90% of the silver. A double or multiple roast is beneficial over a'single roast in that the mixture of coal and sulphur can be proportioned to give different temperatures during successive roasts. This will also permit a wider variation in the amount of air supplied to the furnace.

The sulphur required in a chloridizing roast, had best be in the form of what may be termed fire sulphur, that is, sulphur in combination as pyrite or chalco-pyrite. The nature of sulphur in, this form is, that when once it is ignited,'the combustion will continue without the application of heat from an external source.

When subjecting ores to a chloridizing roast, the two different classes to which attention has already been called, must be considered. One class of ores is characterized by having a gangue of quartz and the like, while the second class of ores is distinguished by a gangue of limestone, talc, spar or by minerals containing magnesia; in fact, most substances containing CO might be included.

The gangue of the first mentioned class of ores may be said to be wholly inactive or inert, so far as having any effect on the chloridizing roast is concerned. Not so, however, is the gangue of the second kind,

for on the contrary, this kind may be said to be highly active and therefore plays an important part by its effect on the action of the chloridizing roast. The consequence of this activity is the taking up by the gangue, of a considerable part of the metal sulphides and sodium chloride during the first roast, as for instance, Ca(Mg)CO is converted into Ca(Mg)O, Ca(Mg) SO, and Ca(Mg) C1 thus depriving the valuable metallic contents of the ore, of the full benefit of the chloridizing roast, and resulting in a lessened extraction of the metals as previously mentioned herein.

In considering the advantages of the present process, it is well to bear in mind some of the disadvantages of single-roast chloridizing with relation toores of the second kind. In a single roast, it is impossible to add to the ore being treated, a

sintered ore. -Such procedure would, of

course, be absurd and prohibitive.

Now, to give directly, some of the advantages of the multiple-roast chloridizing, it

may be stated that in this process, the amount of sulphur necessary to secure the full extraction of the values, can be divided between successive roasts, so that in each separate roast there will not be sulphur enough to"cause an excessive temperature, therefore eliminating any danger of sinter ing, and leaving the charge after the final roasting, in a soft or loose condition so that it can be easily and quickly discharged from the furnace, and permitting the uninterru ted operation of the furnace.

ulphur in combination with lead, as galena and the like, can be used in a chloridizing roast, providing suflieient coal, or other combustible substance, is added to the charge to maintain the combustion and thus decompose the lead sulphid. When the ore to be treated, contains large quantities of sulphur, a part, or all, of the sulphur may be removed by giving the ore an oxidizing roast previous to the chloridizing roast. In such cases, certain quantities of the same high sulphur ore may be used with the previousl the necessary sulpli roasts.

In many cases coal does not have to be added to the furnace charge at all, as the entire combustion can be sustained by the ur for the chloridizing sulphur content alone.

The present process is not concerned with particular ingredients, per se, nor with the percentages of the ingredients, per se, its sole purpose being to apply a series of successive chloridizing roasts, and in distributing the total chloridizing effect through the difi'er ent roasts in such proportions asmay be deemed most satisfactory in each particular instance.

Many different modifications of the chlorination process have been evolved for treating different ores, and for operating under different conditions, but all the different methods heretofore employed, are based on giving the oreonly one chloridizing roast. It is immaterial to my discovery, just which modification is used, so long as better results are obtained by employing more than one chloridizing roast.

A partial or incomplete chloridization may be said to take place in each roasting stage, with the completion thereof in the final roasting stage.

Between the succeeding roasting stages the ore may be cooled more or less.

It will be understood, that wherever the term salt is used herein, sodium chloride (NaCl) is meant.

While the description of the present process as embodied in this specification, is based on various experiments and on numerous tests under actual working conditions, I wish it understood that more or less theory, ad-

oxidized ore to furnish vanced by myself during the progress of the work, is involved inmy explanation, and it is not my purpose thereby to define or limit -my rights as to the beneficial effects which are obtainable by the use of this discovery.-

Furthermore, the various details as to the practice of my improved process, may, or may not, be given in the preferred forms in the above recitation, and also, the preferred forms may be varied from time to time, depending upon the circumstances and conditions governin ,and governed by, the development of t is discovery and the arts to which it is incident, so the Interested inquirer is directed to interpret the scope of this discovery, from the claims, in which. its spirit is broadly generalized.

Having fully described my discovery, what I claim is:

1. In a multiple-roast chloridizing process for. ore, the method of chloridizing the values in the said ore, consisting in subjecting the ore to a plurality of successive chloridizing roasts, the said ore being cooled between succeeding roasts.

2. In a multiple-roastchloridizing proc ess for ores, the method of roasting the said ores in a series of succeeding stages, suitable quantities of salt, sulphur and coal mixed with the said ores at each of the said roasting stages.

being 3. In a multiple-roast chloridizing procproduct of a previous roasting, and again roasting each new mixture, until the desired degree of extraction of the valuable contents of the said ores has been reached.

4. A multiple-roast chloridizing process for ores, the said process bein characterized by an initial roasting stagew erein the furnace charge comprises a mixture of ore, salt, sulphur and coal; and one or more subsequent roasting stages, wherein the furnace charge of each stage comprises a mixture of the product of a previous roasting stage with fresh quantities of salt, sulphur and coal; the amount of sulphur employed in each stage being proportioned to produce,

on combustion, a temperature below that which would cause a sintering of the corresponding furnace charge.

5. A multiple-roast chloridizing process for ores, the said process being practiced in two or more roasting stages, each stage embracing the mixing of the ores or the product of a previous roasting stage, as the case may be, with suitable quantities of 'salt, sulphur and coal, and then roasting the mix- 10 phur' and coal, and then roasting the mixture ture in the presence of air, the quantity of in the presence of air. sulphur employed in eaoh of the said stages 6. A multiple-roast chloridizing process being so proportioned that on combustion of 6 for ores, the said process being practiced in the sulphur, a sintering of the mixture shall two or more roasting stages, each stage emnot take place. 15 bracing the mixing of the ores or the prod- In testimony whereof, I sign my name not of a previous roasting stage, as the case hereto. may be, with suitable quantities of salt, sul- OSCAR B. HOFSTRAND. 

